Increasing the mechanical stability of formaldehyde-preserved natural rubber latex



Patented Feb. 26, 1952 INCREASING THE MECHANICAL STABILITY OF FORMALDEHYDE-PRESERVED NATU- RAL RUBBER LATEX Edward M. Bevilacqua, Ramsey, N. J., asslgnor to United States Rubber Company, New York, N. Y., a corporation of New Jersey N6 Drawing. Application February 2, 1950, Serial No. 142,084

8 Claims.

This invention relates to increasing the mechanical stability of formaldehyde-preserved natural rubber latex. v

Natural Hevea rubber latex is commonly preserved today with ammonia. It is also known to preserve latex with formaldehyde, and this has the advantage that the solid rubber derived from formaldehyde-preserved latex is much softor than the rubber derived from ammonia-preserved latex, and this is desirable in many manufacturing processes. Rubber from ammoniapreserved latex has a Mooney viscosity of around 115 to 160, and rubber from formaldehyde-preserved latex has a Mooney viscosity of around 58 to 69, as measured at 100 C. by the Mooney shearing disc plastometer described by M. Mooney in Industrial and Engineering Chemistry, Anal. Ed. 6, 147 (1934). However, such formaldehydepreserved latex thickens rapidly with age, and

in a few months becomes too thick for direct use in manufacturing processes. Volatile secondary and tertiary amines have been added .to formaldehyde-preserved latex to prevent this thickening on ageing, and to give a formaldehydepreserved latex which will remain fluid over a long period of time. However, there is still a serious disadvantage to the use of such latices in manufacturing processes because of their very low mechanical stability. The mechanical stability of formaldehyde-preserved latices, including those where there has also been added a volatile amine to maintain the requisite fluidity over a long period of time, is much lower than the mechanical stability of ammonia-preserved latices, and is too low for many commercial usages. Some conventional surface-active a ents will effectively increase the mechanical stability of formaldehyde-preserved latices when used in amounts of 1% or more, but-they are ineffective at low concentrations, as up to 0.2%. Other conventional surface-active agents will coagulate formaldehyde-preserved latices when used in amounts of 1% or more. All percentages and parts referred to herein are by weight. Such large amounts as 1% of added surface-active -agents which will increase the mechanical stability of the latex are commercially impractical,

:and may impart various difficulties to the use of the latexin manufacturing operations and introduce undesirable properties to the final rubber products made from the latex. 1

The object of the present invention is to increase effectively the mechanical stability of formaldehyde-preserved latex without adding more than 0.2% by weight of a material based on the latex. The terms natural rubber latex and latex are used herein to designate the latex of the Hevea brasiliensis tree, and unless otherwise specified include normal and concentrated Heoea brasiliensis latex.

According to the present invention, the mechanical stability of formaldehyde-preserved latex is effectively increased by incorporating in the latex a small amount of methyl cellulose.

' In carrying out the present invention, 0.01% to 0.2%, based on the latex, of methyl cellulose is incorporated in the formaldehyde-preserved latex, which may benormal or concentrated. The amount of formaldehyde that is added to the freshly tapped latex may be from 0.1 to 1%. The addition of the formaldehyde to the freshly tapped latex reduces the'pI-I from about '7 to a value in the range 5.5 to 6;8 depending on the amount of formaldehyde used. The formaldehyde is distributed throughout the serum of the uncoagulated latex, and on removal of serum in the concentration of the latex as by centrifuging or chemical creaming, the formaldehyde content of the cream may be lowered to as little as 0.03%. Further formaldehyde may be added to the concentrated latex if desired. For increasing the fluidity of the latex on standing, it is sometimes desirable to also add 0.1 to 1%, based on the latex, of a volatile, saturated, secondary or tertiary amine, e. g., dimethylamine, trimethylamine, diethylamine, triethylamine, and 'morpholine. Such volatile amine may be added to the unconcentrated or concentrated latex, or may be added both before and after concentration. The pH of such latices'containing about 0.03 to 1% of formaldehyde and 0.1 to 1% of volatile secondary or tertiary amine will generally be in the range of 6 to 10.5. Such volatile secondary and tertiary amines do not react in the latex with the formaldehyde as do ammonia or primary amines, and hence the latices retain the desirable property of yielding the low viscosity rubber, of Mooney 58 to 69, characteristic of formaldehyde-preserved latices, although they are still low in mechanical stability. On the other hand, the preservation of latex by the addition of formaldehyde and ammonia in amount in excess of that required to react with the formaldehyde, as in U. S. Patent 1,872,161, gives a latex of high mechanical stability, but such a latex is essentially an ammonia-preserved latex and the rubber has the usual high Mooney viscosity of around to 160.

Methyl cellulose is made by the-etherification of some of the hydroxyl groups in cellulose, and

is produced in various viscosity grades containing from 1.4 to 2.1 methoxyl radicals per glucose unit. Methyl cellulose having viscosities in the known wide range of to 10,000 centipoises (cps) measured in 2% aqueous solutions at 20 C. may be used in the present invention. In that methyl cellulose becomes insoluble in water at elevated temperatures, as above 50 C., it should not be used to increase the mechanical stability of formaldehyde preserved latices at temperatures above 50 C.

The methyl cellulose may be added to the latex at the plantations when the latex is initially treated with formaldehyde, with or without a volatile secondary or tertiary amine, or it may be added to such formaldehyde-preserved latex after preservation and before shipment from the plantations, or it may be added to the formaldehyde-preserved latex after arrival in the country to which it is exported. If the latex is to be concentrated, the methyl cellulose will generally be added after concentration to prevent waste of the material. The formaldehyde-preserved latex, which may or may not contain additional volatile secondary or tertiary amine, will have sufiicient mechanical stability to withstand shipment, but it should have its mechanical stability eifectively increased for use in manufacturing processes. The addition to the latex of up to 0.2% of methyl cellulose has no significant effeet on properties of the latex other than effectively to increase its mechanical stability, and in the case of the higher viscosity grades of methyl cellulose, to increase the viscosity of the latex. The coagulation of the latex by salt or by acid is not hindered. Cured films of rubber from the latex are indistinguishable from those made without addition of methyl cellulose.

In the work to be described below illustrating the invention, the mechanical stability of the latices was measured by the method described in Examination of-RubberLatex and Rubber Latex Compounds by Jordan, Brass, and Roe, Ind. and Eng. Chem. 9, 182-198, the particular test for Mechanical Stability being found on pages 188 and 189. In the mechanical stability determinations, fifty milliliters of latex in a jacketed stainless steel vessel 3.8 cm. square were stirred with a propeller blade driven by a high- Example I A latex was preserved in Malaya by adding 0.15% of formaldehyde and 0.3% of'dimethylamine, based on the latex, followed shortly by centrifuging, and then adding a further 0.3% of dimethylamine, based on the concentrated latex, giving a concentrated latex containing about 0.05% of formaldehyde and 0.4% of dimethylamine. The centrifuged latex as imported into the United States had a solids content of 63.5%, a pH of 8.5, and a mechanical stability of 120 seconds. The addition to separate portions of the concentrated latex of 0.05% .ofdifferent methyl 4 celluloses having viscosities of 15, 25, 100, 400, 1500 and 4000 cps. in 2% aqueous solutions at 20 0., increased the mechanical stability to 380, 530, 480, 310, 545 and 750 seconds, respectively.

Example II A latex preserved in Malaya with 0.4% of formaldehyde was imported into the United States where it was found to have a solids concentration of 37.3%, a pH of 5.8, and a mechanical stability of seconds. The addition to the latex of 0.05% of a methyl cellulos having a viscosity of 400 cps. in 2% aqueous solution at 20 C., increased the mechanical stability to 310 seconds.

Example III A latex was preserved in Malaya with 0.15% of formaldehyde and 0.3% of morpholine, and was then centrifuged. To the centrifuged product Were added 0.3% of formaldehyde and 0.39% of morpholine, and the concentrate was imported into the United States where it was found to have a solids concentration of 63.6%, a pH of 6.7, and a mechanical stability of 88 seconds. The addition to the concentrated latex of 0.05% of a methyl cellulose having a viscosity of 400 cps. in 2% aqueous solution at 20 0. increased the mechanical stability to 245 seconds.

Example IV A latex was preserved in Malaya with 0.4% of formaldehyde and 0.2% of trimethylamine. It was imported into the United States where it was centrifuged, followed by addition of 0.12% of tri methylamine based on the concentrated latex. The concentrated latex had a pH of 8.5, a solids concentration of 60.5%, and a mechanical stability of 55 seconds. The addition to the concentrated latex of 0.05% of a methyl cellulose having a viscosity of 400 cps. in 2% aqueous solution at 20 C. increased the mechanical stability to 250 seconds.

In general, conventional anionic, cationic or non-ionic surface-active agents containing long chain hydrophobic groups do not effectively increase the mechanical stability of formaldehydepreserved latices when added in the small amounts (0.01 to 0.2% based on the latex) used with methyl cellulose according to the present invention. For example, 0.05% of the following surface-active agents containing hydrophobic groups, when added to the latex of Example I, gave mechanical stabilities of less than 200 seconds, which shows the ineffectiveness of such surface-active agents to raise the mechanical stability as compared with the methyl cellulose of the present invention: dioctyl ester of sodium sulfosuccinic acid, sodium sulfate derivative of I-ethyl-Z-methyl-undecanol-,

alkyl naphthalene sodium sulfonates, condensation product of formaldehyde with sodium naphthalene sulfonate, stearyl dimethyl benzyl ammonium chloride, benzyl triethyl ammonium chloride, nonaethylene glycol oleate and laurate and stearate, monoand poly-esters of sorbitan and long chain fatty acids, e. g. sorbitan monolaurate and monoleate and monostearate and sorbitan trioleate and tristearate, reaction products of ethylene oxide with such monoand poly-esters of sorbitan and long chain fatty acids condensation products of ethylene oxide with oleic acid and with oleyl alcohol and with tall oil acids.

Carboxymethyl cellulose .and hydroxyethyl eel.-

lulose are relatively ineifective for increasing the mechanical stability of formaldehyde-preserved latex. For example, 0.05% of various carboxymethyl cellulose of diiferent viscosities added to the latex used in Example I which had a mechanical stability of 120 seconds gave mechanical stabilities of less than 200 seconds. Also, 0.1% of various hydroxyethyl celluloses of different viscosities added to the latex used in Example I gave mechanical stabilities of less than 200 seconds.

In view of the many changes and modifications that may be made without departing from the principles underlying the invention, reference should be made to the appended claims for an understanding of the scope of the protection afforded the invention.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. The method of increasing the mechanical stability of formaldehyde-preserved natural Hevea rubber latex having a pH between 5.5 and 6.8 which comprises incorporating in the latex 0.01 to 0.2% of methyl cellulose at a temperature below 50 C. 2. The method of increasing the' mechanical stability of natural Hevea rubber latex containing 0.03 to 1% of formaldehyde which comprises incorporating in said latex 0.01 to 0.2% of methyl cellulose at a temperature below 50 C.

3. The method of increasing the mechanical stability of natural Hevea rubber latex containing 0.03 to 1% of formaldehyde and 0.1 to 1% of amine of the group consisting of dimethylamine, trimethylamine, diethylamine, trithyla mine, and morpholine, which comprises incorporating in said latex 0.01 to 0.2% of methyl cellulose at a temperature below 50 C.

4. The method of increasing the mechanical stability of natural Hevea rubber latex containing 0.03 to 1% of formaldehyde and 0.1 to 1% of morpholine which comprises incorporating in said latex 0.01 to 0.2% of methyl cellulose at a temperature below C.

5. A formaldehyde-preserved natural Hevea rubber latex having a pH between 5.5 and 6.8 containing 0.01 to 0.2% of methyl cellulose and being at a temperature below 50 C.

6. A natural Hevea rubber latex containing 0.03 to 1% of formaldehyde and 0.01 to 0.2% of methyl cellulose and being at a temperature below 50 C.

7. A natural Hevea rubber latex containing 0.03 to 1% of formaldehyde, 0.1 to 1% of an amine Of the group consisting of dimethylamine, trimethylamine, diethylamine, triethylamine, and morpholine, and 0.1 to 0.2% of methyl cellulose and being at a temperature below 50 C.

8. A natural Hevea latex containing 0.03 to 1% of formaldehyde, 0.1 to 1%-'of morpholine, and 0.01 to 0.2% of methyl cellulose and being at a temperature below 50 C.

EDWARDM. BEVILACQUA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,872,161 McGavach Aug. 16, 1932 1,994,328 Teflt Mar. 12, 1935 2,138,073 Schweitzer Nov. 29, 1938 2,327,115 Linscott Aug. 17, 1943 2,427,618 Nagle et al Sept. 16, 1947 

1. THE METHOD OF INCREASING THE MECHANICAL STABILITY OF FORMALDEHYDE-PRESERVED NATURAL HEVEA RUBBER LATEX HAVING A PH BETWEEN 5.5 AND 6.8 WHICH COMPRISES INCORPORATING IN THE LATEX 0.01 TO 0.2% OF METHYL CELLULOSE AT A TEMPERATURE BELOW 50* C. 